A construction machine, such as a hydraulic excavator, generally includes a diesel engine as a prime mover, and performs necessary work by rotationally driving at least one variable displacement hydraulic pump by the diesel engine and driving hydraulic actuators with a hydraulic fluid delivered from the hydraulic pump. The diesel engine is provided with an input means, e.g., accelerator lever, for commanding a target revolution speed. The fuel injection volume is controlled in accordance with the target revolution speed, whereby the engine revolution speed is controlled.
For such control of the engine and the hydraulic pump in the hydraulic construction machine, the so-called speed sensing control has hitherto been performed through the steps of determining the difference (revolution speed deviation) between the target revolution speed and an actual engine revolution speed outputted from a revolution speed sensor, and controlling an input torque of the hydraulic pump based on the revolution speed deviation. The speed sensing control is intended to reduce a load torque (input torque) of the hydraulic pump when the detected actual engine revolution speed is lower than the target revolution speed, thereby effectively utilizing the engine output while preventing stalling of the engine.
The engine output greatly changes depending on environments around the engine. When the hydraulic construction machine is used in, e.g., highland, an engine output torque reduces with lowering of the atmospheric pressure. JP,A 11-101183, for example, discloses the prior art capable of responding to changes in environments and suppressing a reduction of the engine revolution speed even when the engine output is reduced.
The disclosed prior art comprises a prime mover, a variable displacement hydraulic pump driven by the prime mover, a fuel injection device (governor) for controlling fuel injection in the prime mover, input means (target engine revolution speed input unit) for commanding a target revolution speed of the prime mover, revolution speed detecting means (revolution speed sensor) for detecting an actual revolution speed of the prime mover, a controller for controlling a maximum absorption torque of the hydraulic pump based on the target revolution speed commanded from the input means and the actual revolution speed detected by the revolution speed detecting means, and a plurality of sensors (e.g., an atmospheric pressure sensor and a fuel temperature sensor) for detecting various status variables (e.g., an atmospheric pressure and a fuel temperature) related to the environments of the prime mover and outputting corresponding detected signals for the respective status variables.
Further, in the prior art, the controller includes a torque modification value computing unit for modifying the maximum absorption torque of the hydraulic pump in accordance with the detected signals for the status variables. The controller previously stores tables, in number corresponding to the various sensors, for computing modification gains corresponding to the detected signals from the various sensors, and the torque modification value computing unit computes a torque modification value after applying predetermined weights to the modification gains computed based on the respective tables. Then, the controller sets, as a final target maximum absorption torque, the maximum absorption torque of the hydraulic pump, which has been modified by using the modified torque modification value, and then outputs the final target maximum absorption torque, as a command current value, to a corresponding solenoid valve.